First, a MIMO technology applied to the present invention will be briefly described.
The MIMO is abbreviated from the term “multi-input multi-output” and indicates a method of employing multiple transmission antennas and multiple reception antennas so as to improve transmission/reception data efficiency, instead of a conventional method using one transmission antenna and one reception antenna. That is, a transmitter or a receiver of a radio communication system uses multiple antennas so as to increase communication capacity or improve transmission/reception performance.
The MIMO technology indicates a technology of collecting data pieces received via several antennas without depending on a single antenna path in order to receive one message. According to the MIMO technology, a data transmission rate is improved in a specific range or a system range can be increased with respect to a specific data transmission rate. That is, the MIMO technology is the next-generation mobile communication technology which is widely applicable to mobile communication terminals and repeaters.
The MIMO technology is attracting attention as the next-generation technology to overcome the restricted transmission amount of the mobile communication that has reached the limit due to the data communication extension. Since the next-generation mobile communication requires a data transmission rate significantly higher than that of the existing mobile communication, the efficient MIMO technology is expected to be required.
Among various technologies of improving transmission efficiency which are currently being researched, the MIMO technology of using multiple antennas in both a transmitter and a receiver is attracting most attention as a method of remarkably improving communication capacity and transmission/reception performance with increasing additional frequency allocation or power consumption.
FIG. 1 is a view showing the configuration of a general MIMO antenna system.
As shown in FIG. 1, if the number of antennas is increased in both a transmitter and a receiver, a theoretical channel transmission capacity is increased in proportion to the number of antennas, unlike the case where multiple antennas are used in only one of the transmitter and the receiver. Thus, frequency efficiency can be remarkably improved.
After the increase in the theoretical capacity of the MIMO system was proved in the mid-1990s, various technologies of substantially improving a data transmission rate have been actively developed up to now. Among them, several technologies are already applied to the various radio communication standards such as the third-generation mobile communication and the next-generation wireless local area network (LAN).
According to the researches into the MIMO antenna up to now, various researches such as researches into information theory related to the computation of the communication capacity of a MIMO antenna in various channel environments and multiple access environments, researches into the model and the measurement of the radio channels of the MIMO system, and researches into space-time signal processing technologies of improving transmission reliability and transmission rate have been actively conducted.
The MIMO technology includes a spatial diversity method for increasing transmission reliability using symbols passing through various channel paths and a spatial multiplexing method for improving a transmission rate by simultaneously transmitting a plurality of data symbols using a plurality of transmission antennas. Recently, researches into a method of obtaining the respective advantages of the two methods by combining the two above-described methods are ongoing.
Hereinafter, the methods will be described in detail.
First, the spatial diversity method includes a space-time block coding method and a space-time trellis coding method using both a diversity gain and a coding gain. Generally, the trellis coding method is excellent in view of the improvement of a bit error rate and the degree of freedom for code generation, but the space-time block coding method is advantageous in that computation complexity is simple. A spatial diversity gain can be obtained from a product of the number of transmission antennas and the number of reception antennas. Instead of the space-time coding method, a “space-frequency coding method” based on a frequency domain instead of a time domain may be used. At this time, the same coding method may be applied without change.
Second, the spatial multiplexing method indicates a method of transmitting different data streams via transmission antennas. At this time, in a receiver, mutual interference is generated between data which are transmitted from a transmitter. The receiver eliminates the interference using an adequate signal processing method and receives the data. The receiver for eliminating noise, which is used herein, includes a maximum likelihood receiver, a zero forcing (ZF) receiver, a minimum mean-squared errors (MMSE) receiver, a Diagonal Bell Laboratories Layered Space-Time (D-BLAST) receiver and a Vertical Bell Laboratory Layered Space-Time (V-BLAST) receiver. In particular, if the transmitter knows channel information, a singular value decomposition (SVD) method may be used.
Third, a combination of the spatial diversity method and the spatial multiplexing method may be used. If only the spatial diversity gain is obtained, a performance improvement gain according to the increase in diversity order is gradually saturated. If only the spatial multiplexing gain is obtained, the transmission reliability of the radio channel deteriorates. Accordingly, researches into the methods of obtaining both the two gains while solving the above-described problems have been conducted. Among them, a Double Space-Time Transmit Diversity (Double-STTD) or Space-Time Bit Interleaved Coded Modulation (STBICM) may be used.
Hereinafter, a method of using an Alamouti code for transmission diversity in the MIMO system will be described.
In a transmission diversity (hereinafter, abbreviated to “Tx Div”) structure using the Alamouti code, if the receiver is changed to an equivalent receiver shown in FIG. 2, the receiver may be easily analyzed.
FIG. 2 is a view showing the structure of the equivalent receiver in order to facilitate the description of the system using the Alamouti code.
That is, if a conjugate complex number is applied to a second receiving signal for mathematical modeling of the receiving signal in the diversity transmission method using the Alamouti code, more efficient mathematical modeling is possible. FIG. 2 shows that the receiving signal can be expressed in a matrix form by applying the conjugate complex number to the receiver at a time 2 or a frequency 2. Hereinafter, the detailed mathematical modeling using the conjugate complex number will be described.
As the matrix of the Alamouti code, two matrixes can be considered as follows.
                    [                                                            s                1                                                                    -                                  s                  2                  *                                                                                                        s                2                                                                    s                1                *                                                    ]                            Equation        ⁢                                  ⁢        1                                [                                                            s                1                                                                    s                2                                                                                        -                                  s                  2                  *                                                                                    s                1                *                                                    ]                            Equation        ⁢                                  ⁢        2            
In the matrixes of Equation 1 and Equation 2, each column denotes a time or a frequency and each row denotes the antenna. In more detail, the matrix of Equation 1 indicates a general equation of a matrix which is first suggested in the paper of Alamouti, and Equation 2 indicates a matrix used in the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) standard. That is, Equation 2 indicates a matrix which is reconfigured such that the signal transmitted via an antenna 1 in a single-input single-output (SISO) system using only one antenna is equal to the signal transmitted via the antenna 1 in an Alamouti system.
In the above-described equations, if each column denotes the time, the Alamouti code is used as a kind of a space time block code (STBC) and, if each column denotes the frequency, the Alamouti code is used as a kind of a space frequency block code (SFBC).
Now, the detailed description will be made with reference to FIG. 2.
When the transmission diversity of the Alamouti system is used, the receiving signal may be expressed as follows. Hereinafter, the STBC that the columns of Equation 1 and Equation 2 denote the time will be described. However, the same mathematical modeling is applicable to the SFBC that the columns of Equation 1 and Equation 2 denote the frequency. If the signals of a time 1 and a time 2 are expressed by y1 and y2, the signals y1 and y2 are expressed by the following equation.
                    {                                                                              y                  1                                =                                                                            h                      1                                        ⁢                                          s                      1                                                        +                                                            h                      2                                        ⁢                                          s                      2                                                        +                                      n                    1                                                                                                                                            y                  2                                =                                                                            -                                              h                        1                                                              ⁢                                          s                      2                      *                                                        +                                                            h                      2                                        ⁢                                          s                      1                      *                                                        +                                      n                    2                                                                                                          Equation        ⁢                                  ⁢        3            
In Equation 3, n1 and n2 denote noises generated in the reception antennas, s1 and s2 denote the transmitting signals at the time 1 and the time 2, and h1 and h2 denote the transmission channel values of the antennas.
At this time, as shown in FIG. 2, if a conjugate complex number is applied to the receiving signal of the second time, the signals are expressed by the following equation.
                    {                                                                              y                  1                                =                                                                            h                      1                                        ⁢                                          s                      1                                                        +                                                            h                      2                                        ⁢                                          s                      2                                                        +                                      n                    1                                                                                                                                            y                  2                  *                                =                                                                            -                                              h                        1                        *                                                              ⁢                                          s                      2                                                        +                                                            h                      2                      *                                        ⁢                                          s                      1                                                        +                                      n                    2                                                                                                          Equation        ⁢                                  ⁢        4            
If the model of the receiving signal of Equation 4 is expressed by a vector and a matrix, the following equation is obtained.
                                          [                                                                                y                    1                                                                                                                    y                    2                    *                                                                        ]                    =                                                    [                                                                                                    h                        1                                                                                                            h                        2                                                                                                                                                h                        2                        *                                                                                                            -                                                  h                          1                          *                                                                                                                    ]                            ⁡                              [                                                                                                    s                        1                                                                                                                                                s                        2                                                                                            ]                                      +                          [                                                                                          n                      1                                                                                                                                  n                      2                                                                                  ]                                      ⁢                                  ⁢                  y          =                                                    H                eff                            ⁢              s                        +            n                                              Equation        ⁢                                  ⁢        5            
where, Heff denotes an efficient channel.
If channel estimation is complete, the decision statistic z of the receiving signal expressed by the above-described equation is as follows. That is, in the receiving process, the receiving signal is multiplied by Hermitian of the efficient channel Heff.
                                                        z              =                            ⁢                                                H                  eff                  H                                ⁢                y                                                                                        =                            ⁢                                                                    H                    eff                    H                                    ⁢                                      H                    eff                                    ⁢                  s                                +                                                      H                    eff                    H                                    ⁢                  n                                                                                                        =                            ⁢                                                                                                                h                                                              F                    2                                    ⁢                  s                                +                                  n                  ^                                                                                        Equation        ⁢                                  ⁢        6            
where, it is noted that HeffHHeff=∥h∥F2I and Var({circumflex over (n)})=∥h∥F2σ2I. In addition, I denotes a unit matrix and ∥ ∥F2 denotes a Frobenius norm (I2 norm).
As a result, a decision variable may be expressed as follows.zi=∥h∥F2si+{circumflex over (n)}i where I=1,2  Equation 7
A signal to interference plus noise ratio (SINR) of the decision variable may be expressed by the following equation.
                    ρ        =                                                                              (                                                                                  h                                                              F                    2                                    )                                2                            ⁢                              E                s                                                                                                        h                                                  F                2                            ⁢                              σ                2                                              =                                                                    h                                            F              2                        ⁢                                          E                s                                            σ                2                                                                        Equation        ⁢                                  ⁢        8            
When the receiving signal is multiplied by the Hermitian of the efficient channel in the receiving process, channel estimation error is considered. In a range in which the generality representing the effect of the channel estimation error is not lost, for convenience of computation, only phase error is considered and a magnitude is not considered in view of the channel estimation error. In addition, a noise signal is ignored.
                                                                        [                                                                                                                              s                          ^                                                1                                                                                                                                                                          s                          ^                                                2                        *                                                                                            ]                            =                            ⁢                                                                    [                                                                                                                                                      h                              1                              *                                                        ⁢                                                          ⅇ                                                              -                                                                  jθ                                  1                                                                                                                                                                                                                                                        h                              2                                                        ⁢                                                          ⅇ                                                              jθ                                2                                                                                                                                                                                                                                                                    -                                                              h                                2                                *                                                                                      ⁢                                                          ⅇ                                                              -                                                                  jθ                                  2                                                                                                                                                                                                                                                        h                              1                                                        ⁢                                                          ⅇ                                                              jθ                                1                                                                                                                                                                          ]                                    ⁡                                      [                                                                                                                        h                            1                                                                                                                                -                                                          h                              2                                                                                                                                                                                                        h                            2                            *                                                                                                                                h                            1                            *                                                                                                                ]                                                  ⁡                                  [                                                                                                              s                          1                                                                                                                                                              s                          2                          *                                                                                                      ]                                                                                                        =                            ⁢                              [                                                                                                                                                          (                                                                                                                            h                                  1                                                                ⁢                                                                  h                                  1                                  *                                                                ⁢                                                                  ⅇ                                                                      -                                                                          jθ                                      1                                                                                                                                                                  +                                                                                                h                                  2                                                                ⁢                                                                  h                                  2                                  *                                                                ⁢                                                                  ⅇ                                                                      jθ                                    2                                                                                                                                                        )                                                    ⁢                                                      s                            1                                                                          +                                                                              (                                                                                                                            -                                                                      h                                    1                                    *                                                                                                  ⁢                                                                  h                                  2                                                                ⁢                                                                  ⅇ                                                                      -                                                                          jθ                                      1                                                                                                                                                                  +                                                                                                h                                  1                                  *                                                                ⁢                                                                  h                                  2                                                                ⁢                                                                  ⅇ                                                                      jθ                                    2                                                                                                                                                        )                                                    ⁢                                                      s                            2                            *                                                                                                                                                                                                                                                        -                                                          (                                                                                                                                    -                                                                          h                                      1                                                                                                        ⁢                                                                      h                                    2                                    *                                                                    ⁢                                                                      ⅇ                                                                          jθ                                      1                                                                                                                                      +                                                                                                      h                                    1                                                                    ⁢                                                                      h                                    2                                    *                                                                    ⁢                                                                      ⅇ                                                                          -                                                                              jθ                                        2                                                                                                                                                                                                        )                                                                                ⁢                                                      s                            1                                                                          +                                                                              (                                                                                                                            h                                  1                                                                ⁢                                                                  h                                  1                                  *                                                                ⁢                                                                  ⅇ                                                                      jθ                                    1                                                                                                                              +                                                                                                h                                  2                                                                ⁢                                                                  h                                  2                                  *                                                                ⁢                                                                  ⅇ                                                                      -                                                                          jθ                                      2                                                                                                                                                                                            )                                                    ⁢                                                      s                            2                            *                                                                                                                                              ]                                                                                        =                            ⁢                              [                                                                                                                              α                          ⁢                                                                                                          ⁢                                                      s                            1                                                                          +                                                  β                          ⁢                                                                                                          ⁢                                                      s                            2                            *                                                                                                                                                                                                                                                        -                                                          β                              *                                                                                ⁢                                                      s                            1                                                                          +                                                                              α                            *                                                    ⁢                                                      s                            2                            *                                                                                                                                              ]                                                                        Equation        ⁢                                  ⁢        9                                                          ⁢                  where          ⁢                                          ⁢                                          ⁢                      α            ≡                          (                                                                    h                    1                                    ⁢                                      h                    1                    *                                    ⁢                                      ⅇ                                          -                                              jθ                        1                                                                                            +                                                      h                    2                                    ⁢                                      h                    2                    *                                    ⁢                                      ⅇ                                          jθ                      2                                                                                  )                                ⁢                                          ⁢                                          ⁢          and          ⁢                                          ⁢                                          ⁢                      β            ≡                          (                                                                    -                                          h                      1                      *                                                        ⁢                                      h                    2                                    ⁢                                      ⅇ                                          -                                              jθ                        1                                                                                            +                                                      h                    1                    *                                    ⁢                                      h                    2                                    ⁢                                      ⅇ                                          jθ                      2                                                                                  )                                                                      
In Equation 9, ŝi denotes the estimation signal of an ith transmitting signal si and θi denotes the phase error of the ith antenna due to the channel estimation error. In Equation 9, α denotes the gain of a desired signal when the channel estimation error is considered and β denotes the magnitude of an interference signal due to the channel estimation error.
Equation 9 may be expressed as follows.
                                                                        [                                                                                                                              s                          ^                                                1                                                                                                                                                                          s                          ^                                                2                                                                                            ]                            =                            ⁢                              [                                                                                                                                                          (                                                                                                                            h                                  1                                                                ⁢                                                                  h                                  1                                  *                                                                ⁢                                                                  ⅇ                                                                      -                                                                          jθ                                      1                                                                                                                                                                  +                                                                                                h                                  2                                                                ⁢                                                                  h                                  2                                  *                                                                ⁢                                                                  ⅇ                                                                      jθ                                    2                                                                                                                                                        )                                                    ⁢                                                      s                            1                                                                          +                                                                              (                                                                                                                            -                                                                      h                                    1                                    *                                                                                                  ⁢                                                                  h                                  2                                                                ⁢                                                                  ⅇ                                                                      -                                                                          jθ                                      1                                                                                                                                                                  +                                                                                                h                                  1                                  *                                                                ⁢                                                                  h                                  2                                                                ⁢                                                                  ⅇ                                                                      jθ                                    2                                                                                                                                                        )                                                    ⁢                                                      s                            2                            *                                                                                                                                                                                                                                                        (                                                                                                                            -                                                                      h                                    1                                    *                                                                                                  ⁢                                                                  h                                  2                                                                ⁢                                                                  ⅇ                                                                      -                                                                          jθ                                      1                                                                                                                                                                  +                                                                                                h                                  1                                  *                                                                ⁢                                                                  h                                  2                                                                ⁢                                                                  ⅇ                                                                      jθ                                    2                                                                                                                                                        )                                                    ⁢                                                      s                            1                            *                                                                          +                                                                              (                                                                                                                            h                                  1                                                                ⁢                                                                  h                                  1                                  *                                                                ⁢                                                                  ⅇ                                                                      -                                                                          jθ                                      1                                                                                                                                                                  +                                                                                                h                                  2                                                                ⁢                                                                  h                                  2                                  *                                                                ⁢                                                                  ⅇ                                                                      jθ                                    2                                                                                                                                                        )                                                    ⁢                                                      s                            2                                                                                                                                              ]                                                                                        =                            ⁢                              [                                                                                                                              α                          ⁢                                                                                                          ⁢                                                      s                            1                                                                          +                                                  β                          ⁢                                                                                                          ⁢                                                      s                            2                            *                                                                                                                                                                                                                            α                          ⁢                                                                                                          ⁢                                                      s                            2                                                                          -                                                  β                          ⁢                                                                                                          ⁢                                                      s                            1                            *                                                                                                                                              ]                                                                        Equation        ⁢                                  ⁢        10                                where        ⁢                                  ⁢                  α          ≡                      (                                                            h                  1                                ⁢                                  h                  1                  *                                ⁢                                  ⅇ                                      -                                          jθ                      1                                                                                  +                                                h                  2                                ⁢                                  h                  2                  *                                ⁢                                  ⅇ                                      jθ                    2                                                                        )                          ⁢                                  ⁢        and        ⁢                                  ⁢                  β          ≡                      (                                                            -                                      h                    1                    *                                                  ⁢                                  h                  2                                ⁢                                  ⅇ                                      -                                          jθ                      1                                                                                  +                                                h                  1                  *                                ⁢                                  h                  2                                ⁢                                  ⅇ                                      jθ                    2                                                                        )                                                          
That is, if the channel estimation error is generated, the transmitting signal may be changed to Equation 10 and may have an influence on the reception performance of the receiver.